The present invention relates to a process for coating a substrate with a material, such as a metal, giving a polished effect.
In order to achieve the effect of a high polish on object, according to the prior art electroplating with chromium, nickel, or anodizing are used. When this is done, costly pretreatments of the base material, such as surface polishing, are required in order to achieve the desired effect of a high polish. Further-more, some materials cannot be given a lasting high-lustre metallic finish, as in the case, for example, of chromium on aluminum. In addition, certain processes also entail the disadvantage of contributing to considerable environmental damage.
It is known that PVD or CVD processes can be used in conjunction with a wet-lacquer technique in order to achieve metallizing, although the required durability cannot be achieved in those areas that are endangered by corrosion. The mechanical and chemical stability of wet lacquers is not sufficient for coating parts that are highly stressed. Very frequently, corrosion protection leaves a great deal to be desired, and wet lacquering also gives rise to environmental hazards.
It is the task of the present invention to describe a process of the type described in the introduction hereto, by which high-lustre metallizing can be achieved without causing any environmental damage, and by which almost any desired geometry can be achieved at a consistent quality.
This problem has been solved by a process that comprises the following steps:
a) Cleaning of the substrate and/or application of a base coating on the substrate; PA1 b) Coating the cleaned and/or base-coated substrate with the material giving the polished effect, by deposition, optionally with the help of plasma; PA1 c) Application of a top coating by burning on, to which, if required, a protective coating can be applied.
A powder lacquer coating us used as the base coating, and this is burned on at a substrate temperature of 120.degree. to 240.degree. C.; this burning-in lasts for approximately 8 to 30 minutes.
As a consequence, according to the present invention, it is possible to coat substrates that do not exhibit any deformation in the above temperature range. Such substrates can be of metal, ceramic, glass, plastics, and, in particular, fibre-reinforced plastic. The application of the base coat ensures that the substrate surface is flat, i.e., that rough surfaces can be "metallized" without any mechanical processing; the powder lacquer coating that is to be burned on smoothes the surface in such a way that any rough spots the were originally present are covered over.
The powder is preferably a polyester resin compound, with deposition onto the surface being effect electrostatically. After that, the material that gives the polished effect is applied by the plasma process. Aluminum, chromium, titanium, silver, and gold are examples of suitable materials. To this end, the substrate is placed in a reaction chamber, in which the pressure is initially at least 10.sup.-4, preferably 10.sup.-4 to 10.sup.-5 millibar. This means that, in particular, oxygen and nitrogen molecules are removed to the required extent. The reaction chamber is then flooded with a process gas (inert gas of reactive gas) until the pressure is between 1 millibar and 10.sup.-3 millibar. Finally, a glow discharge is triggered, and a plasma results. The material that gives the desired polished effect is then vapourized in this plasma, so that the vapourized metal is deposited onto the substrate that is in the plasma.
The required plasma can be generated either within the reaction chamber by building up an electrical field between an anode (recipient) and a cathode (substrate) by means of DC current or high frequency (kHz - MHz, preferably 13.56 MHz), or outside the reaction chamber by a high-frequency field (GHz, microwave).
If the plasma is generated by high frequency, it must be ensured that the substrate surface is smaller than the recipient surface in order to ensure sufficient polarization of the electrodes.
The coating can also be effected by means of an arc vapourizer, a laser vapourizer, or by cathodic sputtering (single or double cathode). If this type of coating is used, separate generation of the plasma is eliminated, for the plasma is generated by the vapourizing or sputtering process.
After metallizing, a protective coating can be formed in an intermediate step, for example, by plasma polymerization, and the top coating is applied to this. This top coating is comparable to the base layer with respect to structure and production, i.e., in that a powder consisting preferably of a polyester resin compound is deposited electrostatically and then burned on at a temperature range between 120.degree. C. and 240.degree. C. for a period of 8 to 30 minutes.
Finally, a scratch-proof protective coating can be applied, this consisting preferably of a carbon compound.